An Experimental Study of Elastic Electron Scattering from ...

An Experimental Study of Elastic Electron
Scattering from Fluorocarbon Radicals
School of Chemical and Physical Sciences
Faculty of Science and Engineering
Flinders University
South Australia
Jessica R. Brunton B.Sc.(Nanotechnology)Hons.
Thesis submitted for the fulfilment of the degree of
Doctor of Philosophy
January 2011

Contents
Contents ................................................................................................................................. i
Declaration ........................................................................................................................... iv
Summary ............................................................................................................................... v
Acknowledgements ............................................................................................................ vii
Chapter 1 — Introduction ........................................................................................................ 1
1.1 Cross Sections: Definitions and their Importance ................................................... 1
1.2 Plasma Processing .................................................................................................... 7
1.3 Previous Work ........................................................................................................ 10
1.4 The Scope of the Current Investigation ................................................................. 12
1.5 General Information about the Target Species ..................................................... 16
1.5.1 General Information - CF 2 .............................................................................. 17
1.5.2 General Information - CF 3 .............................................................................. 19
1.5.3 General Information - CF 3 I ............................................................................. 20
Chapter 2 — The Apparatus .................................................................................................. 23
2.1 Basic Overview of the Apparatus ........................................................................... 23
2.2 The Gas Handling System ....................................................................................... 29
2.3 Solenoid Actuated Pulsed Nozzle .......................................................................... 30
2.3.1 The Pyrolytic Assembly .................................................................................. 31
2.3.2 Striking the Silicon Carbide Nozzle................................................................. 33
2.4 The Electron Spectrometer .................................................................................... 38
2.4.1 Electron Monochromator .............................................................................. 39
2.4.1.1 Construction, Wiring and Operation ................................................ 39
2.4.1.2 Monochromator Cleaning Procedure .............................................. 48
2.4.1.3 Tuning the Monochromator ............................................................ 49
2.4.2 Analysers ........................................................................................................ 51
2.4.2.1 Construction, Wiring and Operation ................................................ 51
2.4.2.2 Signal Processing .............................................................................. 55
2.5 Time of Flight Mass Spectrometer ......................................................................... 58
2.5.1 TOFMS Construction and Wiring ................................................................... 59
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2.5.2 Ionisation Source: The Nd:YAG Laser/ Frequency Tripling ............................ 63
2.5.3 TOFMS Operation .......................................................................................... 66
Chapter 3 — Experimental Methods ..................................................................................... 69
3.1 Determining Absolute Values – The Normalisation Technique ............................. 69
3.1.1 The relationship between scattered count rates and cross sections ............ 69
3.1.2 Skimmed Supersonic Relative Density Method (SSRDM) .............................. 77
3.1.3 The preferred reference gas - CF 4 .................................................................. 83
3.2 Experimental Procedures ....................................................................................... 86
3.2.1 Measuring the change in pressure and the effective pumping speed .......... 86
3.2.2 Measuring the Electron Scattering Signals .................................................... 90
3.3 The Production of radicals in situ via vacuum flash pyrolysis ................................ 96
3.3.1 Producing CF 2 radicals .................................................................................... 96
3.3.2 Producing CF 3 radicals .................................................................................. 100
3.4 Determining the e + CF 3 elastic DCSs ................................................................... 110
3.4.1 Extracting CF 3 DCSs from the Measured Multi-component DCSs ............... 110
3.4.2 The Elastic DCSs (CF 3 I, C 2 F 6 , I and I 2 ) used to determine the e + CF 3 DCSs from
the measured multi-component DCSs. ........................................................................ 111
3.4 Summary of the uncertainties in the measured differential cross sections ........ 120
Chapter 4 — Results ............................................................................................................ 122
4.1 Differential and Integral Cross Sections for CF 2 ................................................... 122
4.1.1 Electron Impact Elastic Differential Cross Sections ..................................... 122
4.1.2 Electron Impact Elastic Integral Cross Sections ........................................... 138
4.2 Differential and Integral Cross Sections for CF 3 I .................................................. 143
4.2.1 Electron Impact Elastic Differential Cross Sections ..................................... 143
4.2.2 Electron Impact Elastic Integral Cross Sections ........................................... 153
4.3 Elastic Differential and Integral Cross Sections for electrons scattering from a
“mixed” molecular gas beam ........................................................................................... 156
4.4 Differential and Integral Cross Sections for the CF 3 radical ................................. 163
4.4.1 Electron Impact Elastic Differential Cross Sections ..................................... 163
4.4.2 Electron Impact Elastic Integral Cross Sections ........................................... 177
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Chapter 5 — Conclusions and Future Work ........................................................................ 183
5.1 Conclusions .......................................................................................................... 183
5.2 Future Directions ................................................................................................. 187
Appendices ...................................................................................................................... 190
A. Electron impact elastic DCS for atomic iodine ......................................................... 190
B. Electron impact elastic DCS for molecular iodine .................................................... 195
References ....................................................................................................................... 200
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Declaration
I certify that this thesis does not incorporate without acknowledgment any material
previously submitted for a degree or diploma in any university; and that to the best
of my knowledge and belief it does not contain any material previously published or
written by another person except where due reference is made in the text.
…………………………………………….........………...
Jessica R. Brunton
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Summary
This thesis reports the first measurements of elastic electron scattering differential
and integral cross sections for the trifluoromethyl radical (CF 3 ) for incident electron
energies in the range 7-50eV. In order to make those measurements, it was
necessary to first measure elastic cross sections for electron scattering from
iodotrifluoromethane (CF 3 I), in that case over the 10-50eV energy range, and so
those results are also presented. This thesis also includes an experimental
investigation of low energy (2-20eV) elastic electron scattering from
difluoromethylene (CF 2 ), another important radical species.
Chapter 1 consists of some general information about the CF 2 , CF 3 and CF 3 I species,
including an overview of the previous work undertaken in order to better
understand their scattering behaviour. It also incorporates details about the present
motivation for measuring electron scattering phenomena from each of them, along
with the definitions and general importance of both differential and integral cross
sections.
Chapter 2 presents the particulars of the apparatus that was employed for these
measurements. It includes a summary of the design and operation for the three
main components of the apparatus, including the pyrolytic assembly, the electron
spectrometer and the current time of flight mass spectrometer.
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Chapter 3 contains information pertaining to the experimental methods employed
in this study. It includes details relating to our electron scattering data collection
and analysis methods. This chapter also incorporates details related to the in situ
production of both CF 2 and CF 3 , and information about extracting CF 3 cross sections
from the direct measurements of those for “mixed species” gas beams.
Chapter 4 details our present results, beginning with the low energy elastic
differential and integral cross sections for electron scattering from CF 2 . We then
move on to present the differential and integral cross sections of CF 3 I, a necessary
precursor study towards our ultimate goal of investigating electron collisions with
CF 3 . Thereafter we present our “mixed” beam differential cross sections (DCSs),
followed by our extracted CF 3 DCSs and integral cross sections (ICSs). All of our
results, where possible, are compared with the results from current theoretical
investigations; including some so recent that they have not yet been published.
The main findings of this work are summarised in chapter 5, along with a couple of
possible future directions this research might take. Lastly, unpublished theoretical
differential cross section data for atomic and molecular iodine, vital for this current
work, are presented in the appendices.
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Acknowledgements
This project was made possible by the valuable input of many people from both
Flinders University and around the world, and it is my pleasure to acknowledge
them here:
Firstly I would like to thank my supervisor Professor Michael Brunger. He has not
only made the last four years very productive, but also very enjoyable. It has been a
privilege to work for him and his guidance throughout this project has been
invaluable. I would also like extend my thanks to Dr Todd Maddern who served as
my co-supervisor from January 2007- March 2009, not only for sharing his practical
knowledge but for sharing his passion for science and his tenacious attitude
towards problem solving.
Dr Leigh Hargreaves was completing his PhD on this experiment as I began mine,
and then returned to work part time at Flinders University from February-December
2010. I would like to thank Leigh for showing me the ropes and for his continued
support throughout this project.
Furthermore, both the Flinders University Mechanical workshop and the Electronics
workshop have been invaluable throughout the course of my PhD. Their technical
expertise and experience has been without equal. Bob Northeast, Bill Drury, Wayne
Peacock, Mike Mellow, Bruce Gilbert and Greg Hewitt, thank you all for going above
and beyond.
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Travelling further afield, I would like to extend my gratitude to Professor Mu Tao
Lee and Associate Professor Márcio Bettega for providing us with their theoretical
cross sections for elastic scattering from CF 2 and CF 3 I respectively, so that we could
compare them with our data.
I would also like to thank Dr Carl Winstead and Professor Vincent McKoy who have
been associated with our research for several years. Their theoretical cross sections
and useful advice have improved the quality of this thesis.
I would also like to extend my gratitude to Professor Klaus Bartschat and Dr Oleg
Zatsarinny, along with Professors Gustavo García and Fernando Blanco, for
providing us with cross sections central to the completion of this project.
I would like to thank the Centre for Antimatter-Matter Studies and its director
Professor Steve Buckman for giving me the opportunity to present my work at
various conferences throughout my candidature.
Finally, I would like to thank my family for being so supportive of me over this four
year period, and for preventing me from losing touch with the world outside the
laboratory.
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